A conventional electroluminescent device is fabricated in the manner described now. A transparent electrode layer consisting of indium tin oxide (ITO) is deposited on a transparent substrate which is made of a sheet of polyethylene terephthalate or the like. A luminescent layer, a dielectric layer, and a back electrode layer are laminated on the transparent electrode lying on the transparent substrate. These are sealed by transparent moisture-proof film, thus completing the electroluminescent device.
In this prior art technique, it is common practice to use a cyanoethylated resin as a resinous binder for both luminescent layer and dielectric layer. However, this cyanoethylated resin has the disadvantage that it is highly hygroscopic. On the other hand, the electroluminescent material in the luminescent layer is severely deteriorated by intrusion of moisture. Therefore, in order to protect the electroluminescent material against moisture and to improve the durability, it is essential that the electroluminescent device be sealed by moisture-proof film. Consequently, in the prior art technique, the necessity of the moisture-proof film increases the thickness of the electroluminescent device itself accordingly and decreases its flexibility. Because the moisture-proof film must have a mating space along its outer periphery, the luminescent area is smaller than the two-dimensional size of the electroluminescent device. Furthermore, the moisture-proof film is expensive. Hence, the cost to fabricate the electroluminescent device that needs a sealing step is increased.
A first improved technique for dispensing with the moisture-proof film is described by Timex Corporation in U.S. Pat. No. 4,775,964 relating to a luminescent dial on a watch. In this improved technique, epoxy resin is used as the resinous binder for the luminescent layer. This luminescent dial is installed in a watch case that is a confined narrow space and so this technique can be put into practical use. However, if it is used under an exposed state, the moisture resistance and durability are not satisfactorily high. Furthermore, there is room for improvement of the luminescent brightness.
Meanwhile, we have already proposed an improved electroluminescent device in Japanese patent application No. 231709/1993. In particular, a binder consisting of a fluoride resin is used, so that moisture-proof film can be dispensed with. In addition, high luminescent brightness can be obtained. However, it cannot be said that this second improved technique provides complete moisture resistance.